Asthma is an inflammatory disease of the airways characterized by acute, intermittent and recurrent episodes of inflammation that can be induced by a specific allergen. CD4+ T cells contribute to this process by producing the Type 2 cytokines IL-4, IL-5, and IL-13 and inducing B cell production of IgE in response to T cell receptor (TCR) recognition of allergen peptides bound to MHCII molecules on host cells. In both murine models of disease and asthmatic patients, quiescent allergen peptide:MHCII (pMHCII)-specific CD4+ memory T cells can persist in lungs and lymphoid organs long after resolution of inflammation. Upon subsequent exposure to allergen, CD4+ memory T cells rapidly drive asthma-induced immunopathology making these cells attractive targets for allergen-specific immune modulation. Little is known, however, about the differentiation and maintenance of Th2 cells that orchestrate the asthmatic response due to the challenge of tracking small populations of CD4+ T cells that express allergen pMHCII: specific TCRs. To address this lack of knowledge, we produced an MHCII tetramer containing a peptide from the Der p1 protein of the house dust mite (HDM), Dermatophagoides pteronyssinus, the most common cause of atopic asthma. Using this tetramer and a novel magnetic bead-based cell enrichment method of our design, we have found three allergen- specific CD4+ T cell subsets that develop after the induction of allergic airway inflammation. The central hypothesis of this application is that functionally heterogeneous yet synergistic populations of Th2 effector and memory cells develop in response to competition between lineage-defining transcription factors and underlies asthma pathogenesis. The goals of this proposal are to identify the molecular and cellular mechanisms that lead to Th2 effector cell heterogeneity and determine how these cells persist and contribute to asthma pathogenesis in the lung so they can be modified by immunotherapy. This innovative approach could provide the means for preventing allergenic priming of memory Th2 cells or eliminating them through antigen-specific therapies.